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+import { ArgumentOutOfRangeError } from '../util/ArgumentOutOfRangeError';
+import { Observable } from '../Observable';
+import { OperatorFunction } from '../types';
+import { filter } from './filter';
+import { throwIfEmpty } from './throwIfEmpty';
+import { defaultIfEmpty } from './defaultIfEmpty';
+import { take } from './take';
+
+/**
+ * Emits the single value at the specified `index` in a sequence of emissions
+ * from the source Observable.
+ *
+ * <span class="informal">Emits only the i-th value, then completes.</span>
+ *
+ * ![](elementAt.png)
+ *
+ * `elementAt` returns an Observable that emits the item at the specified
+ * `index` in the source Observable, or a default value if that `index` is out
+ * of range and the `default` argument is provided. If the `default` argument is
+ * not given and the `index` is out of range, the output Observable will emit an
+ * `ArgumentOutOfRangeError` error.
+ *
+ * ## Example
+ *
+ * Emit only the third click event
+ *
+ * ```ts
+ * import { fromEvent, elementAt } from 'rxjs';
+ *
+ * const clicks = fromEvent(document, 'click');
+ * const result = clicks.pipe(elementAt(2));
+ * result.subscribe(x => console.log(x));
+ *
+ * // Results in:
+ * // click 1 = nothing
+ * // click 2 = nothing
+ * // click 3 = MouseEvent object logged to console
+ * ```
+ *
+ * @see {@link first}
+ * @see {@link last}
+ * @see {@link skip}
+ * @see {@link single}
+ * @see {@link take}
+ *
+ * @throws {ArgumentOutOfRangeError} When using `elementAt(i)`, it delivers an
+ * `ArgumentOutOfRangeError` to the Observer's `error` callback if `i < 0` or the
+ * Observable has completed before emitting the i-th `next` notification.
+ *
+ * @param index Is the number `i` for the i-th source emission that has happened
+ * since the subscription, starting from the number `0`.
+ * @param defaultValue The default value returned for missing indices.
+ * @return A function that returns an Observable that emits a single item, if
+ * it is found. Otherwise, it will emit the default value if given. If not, it
+ * emits an error.
+ */
+export function elementAt<T, D = T>(index: number, defaultValue?: D): OperatorFunction<T, T | D> {
+  if (index < 0) {
+    throw new ArgumentOutOfRangeError();
+  }
+  const hasDefaultValue = arguments.length >= 2;
+  return (source: Observable<T>) =>
+    source.pipe(
+      filter((v, i) => i === index),
+      take(1),
+      hasDefaultValue ? defaultIfEmpty(defaultValue!) : throwIfEmpty(() => new ArgumentOutOfRangeError())
+    );
+}